Generally, an output torque of an internal combustion engine is increased with an increase in a rotational speed, but a vehicle requires a large drive force at low speed and small drive force at high speed. That is, the vehicle required opposite torque characteristic to that generated by the engine. In addition, optimum operating points of the engine are limited. Therefore, the vehicle having the engine as a prime mover is provided with a transmission to alter a speed ratio according to need so that the engine is allowed to be operated at the optimum operating points to generate drive force by altering the speed ratio of the transmission based on a running condition such as a vehicle speed, an accelerator opening etc. However, given that the transmission is adapted to shift a gear stage stepwise, a thermal efficiency (or fuel economy) is worsened during shifting the gear stage during shifting the gear stage to another stage if the optimum operating speed exists between the gear stages. In order to avoid such disadvantage, a continuously variable transmission has been used in place of the geared transmission.
A belt-driven continuously variable transmission and a toroidal continuously variable transmission are commonly used in vehicles. The belt driven continuously variable transmission is comprised of a belt for transmitting power a pair of pulleys and a belt running on those pulleys to transmit power therebetween. A running radius of the belt is decreased by widening a groove width of the pulley, and increased by narrowing the groove width of the pulley. In turn, the toroidal continuously variable transmission is comprised of a pair of discs being opposed to each other, and a power roller interposed between those discs. Rotational speeds of those discs are differentiated depending on an inclination of a line connecting contact points between the power roller and each discs with respect to a rotational center of the power roller. That is, speed difference between the disc, i.e., a speed ratio is changed from “1” with an increment in the inclined angle (or tilting angle) of the power roller.
The continuously variable transmission of those kinds are adapted to transmit torque utilizing a friction between the pulley and the belt or a friction between the disc and the power roller so that a speed ratio thereof can be altered continuously. Since the friction is a product of a friction coefficient and a vertical load (or a loading in a normal direction) at the contact points of two members, the larger vertical load is required with an increase in the torque to be transmitted. Specifically, in the belt driven continuously variable transmission of vehicle the vertical load is a load of the pulley to clamp the belt. To this end, the required load is established by delivering hydraulic fluid to a hydraulic actuator integrated with the pulley.
A large drive force is required to launch the vehicle, but a required drive force to cruise the vehicle is smaller than that to launch the vehicle. That is, the larger vertical load is required to establish the friction to launch the vehicle. Specifically, in the belt-driven continuously variable transmission, higher hydraulic pressure to clamp the belt is required when launching the vehicle. In order to launch the vehicle promptly, an additional hydraulic device for establishing larger hydraulic pressure is required. Consequently, the larger drive system and the hydraulic system will be enlarged by such additional hydraulic device. In addition, fuel economy will be worsened as a result of establishing high pressure.
Japanese Patent Laid-Opens Nos. 2000-130548, 2004-076876, 2005-308041 describe systems for dealing with the foregoing disadvantages. Japanese Patent Laid-Open No. 2000-130548 describes a system for a vehicle comprised of a gear train connected to an input shaft through a first clutch, and a continuously variable transmission connected to the input shaft through a second clutch. The gear train is configured to establish forward speed stages and a reverse speed stage. A torque is transmitted from a drive gear disposed on an input shaft to a first countershaft, and further transmitted from the first countershaft selectively to a forward gear train and reverse gear train.
A driven gear meshing with the drive gear and a reverse drive gear are fitted onto the first countershaft. An output drive gear and a reverse driven gear are arranged coaxially while being allowed to rotate. A dog clutch is individually disposed between the first countershaft and the output drive gear, and between the output drive gear and the reverse driven gear. the first countershaft is connected to the output drive gear through the dog clutch, or the reverse driven gear is connected to the output drive gear through the dog clutch. A first idle gear is fitted onto a first idler shaft to be meshed with the reverse driven gear disposed on the first countershaft. A second idle gear is disposed on the idler shaft to be meshed with the first idle gear. A third idle gear is disposed on the second idler shaft to be meshed with the reverse driven gear.
The output drive gear is meshed with an output driven gear fitted onto an output shaft while being allowed to rotate. A one-way clutch and a third clutch juxtaposed in parallel each other are disposed between the output drive gear and an output shaft. A secondary pulley of driven side is fitted onto the output shaft of the belt driven continuously variable transmission, and a primary pulley of drive side is fitted onto the input shaft. Those primary and secondary pulleys are connected through a belt running thereon. The input shaft and the primary pulley are connected through the second clutch. According to the Japanese Patent Laid-Open No. 2000-130548, the vehicle is launched in forward direction by engaging the first clutch while connecting the first countershaft to the output drive gear through the dog clutch thereby transmitting torque from the input shaft to the output shaft through the gear train. By contrast, the torque is transmitted from the input shaft to the reverse driven gear and the output drive gear through the first countershaft, the first idler shaft, and the second idler shaft, by engaging the first clutch while connecting the reverse driven gear to the output drive gear through the dog clutch. Consequently, the output shat is rotated in a direction opposite to the direction to propel the vehicle in the forward direction, that is, a reverse stage is established. In addition, the torque is transmitted from the input shaft to output shaft trough the belt-driven continuously variable transmission by engaging the second clutch instead of the first clutch to propel the vehicle in a forward direction while alerting a speed ratio continuously.
Japanese Patent Laid-Open No. 2004-076876 describes a power transmission device in which a torque reversing device comprised of a single pinion planetary gear unit is disposed between an input shaft transmitting power from an engine and a primary pulley of a belt-driven continuously variable transmission. A ring gear of the torque reversing device is connected with the primary pulley to be rotated therewith, and a sun gear is connected with an input shaft. Therefore, a forward stage is achieves by connecting the sun gear with the ring gear by a clutch, and a reverse stage is achieved by fixing a carrier by a brake. In addition, a gear train adapted to establish a speed ratio a larger than a maximum speed ratio of a continuously variable transmission is formed between the input shaft and the output shaft integrated with a secondary pulley. An input gear of the gear train is integrated with the input shaft, and an output gear connected with the input shaft through an idle gear is fitted onto the output shaft while being allowed to rotate. In addition, a one-way clutch and a friction clutch are arranged in series between the output gear and the output shaft.
Accordingly, when launching the vehicle in the forward direction, the clutch for connecting the input shaft with the primary pulley is disengaged while engaging the clutch of the output shaft side, thereby transmitting torque to the output shaft from the input shaft through the gear train, the one-way clutch, and the clutch arranged in series therewith. The maximum speed ratio of the continuously variable transmission is slightly smaller than that of the gear train. In this situation, therefore, the secondary pulley and the output shaft integrated therewith are rotated at a speed higher than the previous speed and a rotational speed of output gear so that the one-way clutch is brought into disengagement. Consequently, the torque is transmitted to the output shaft through the continuously variable transmission. Since the gear train thus transmits the torque when launching the vehicle, a large torque is not applied to the continuously variable transmission when launching the vehicle.
According to the teachings of Japanese Patent Laid-Open No. 2005-308041, an engine power is applied to a sun gear of a single-pinion planetary gear unit serving as a torque reversing device, and the sun gear is connected to an input shaft integrated with a primary pulley of a belt-driven continuously variable transmission through a clutch. An input gear is fitted onto the input shaft through a one-way clutch, and the input gear is connected with a ring gear of torque reversing device. The one-way clutch is adapted to be engaged when the input shaft rotates in a forward direction at higher speed than the input gear situated at the outer circumferential side. An output gear is fitted onto an output shaft integrated with a secondary pulley through another one-way clutch, and an idle gear is disposed between the output gear and the input gear while being meshed therewith. That is, the input gear and the output gear are rotated together in the same direction. A gear ratio (or speed ratio) between the input gear and the output gear is slightly smaller than the maximum speed ratio of the continuously variable transmission comprised of those pulleys and the belt wrapped around those pulleys. Said another one-way clutch is adapted to be engaged when the output shaft rotates in the forward direction at higher speed than the output gear. In addition, a friction clutch is arranged in parallel with said one-way clutch. Further, a brake to fix a carrier of the torque reversing device is arranged to drive the vehicle in the backward direction.
Thus, in the power transmission device taught by Japanese Patent Laid-Open No. 2005-308041, the vehicle is launched in the forward direction by connecting the sun gear with the input shaft by the clutch to transmit the torque to main speed change route comprised of the continuously variable transmission the through the input shaft, and by engaging the one-way clutch to further transmit the torque to a sub speed change route. In this situation, since the speed ratio of the gear train is slightly smaller than the maximum speed ratio of the continuously variable transmission, the output gear rotates at higher speed than the output shaft. Consequently, the one-way clutch at the output shaft side is brought into disengagement so that the torque is transmitted to the drive wheels through the gear train. Therefore, the large torque will not be applied to the continuously variable transmission when launching the vehicle. After launching the vehicle, the speed ratio of the continuously variable transmission is gradually reduced with an increase in the vehicle speed so that a rotational speed of the output shaft integrated with the secondary pulley is raised to that of the output gear situated outer circumferential side of the output shaft, and then further raised with a decrease in the speed ratio. As a result, the one-way clutch of output shaft side is brought into engagement so that the torque is transmitted to the drive wheels through the continuously variable transmission. In this situation, the one-way clutch of the input shaft side is in disengagement, therefore an interlock will not occur.
In any of the teachings of those prior art documents, the gear train is arranged in parallel with the belt-driven continuously variable transmission, and the torque for propelling the vehicle is transmitted through the gear train. Especially, according to Japanese Patent Laid-Open No. 2000-130548, the torque applied to the gear train is selectively transmitted through the dog clutch to the gear train for launching the vehicle and to the gear train for propelling the vehicle in the backward direction. To this end, in the continuously variable transmission taught by Japanese Patent Laid-Open No. 2000-130548 a total of four engagement elements is required such as the first clutch, the second clutch, the dog clutch, and the one-way clutch. In addition, the third clutch arranged in parallel with the one-way clutch is also required. According to the teachings of Japanese Patent Laid-Open No. 2000-130548, therefore, the torque can be transmitted to launch the vehicle and to propel the vehicle in the backward direction without using the belt-driven continuously variable transmission. However, large number of engagement elements is required to establish such torque transmission route. Therefore, the structure of the transmission is complicated and dimension thereof is enlarged.
As described, according to the teachings of Japanese Patent Laid-Open No. 2000-130548, the torque transmission route for propelling the vehicle in the forward direction and the torque transmission route for propelling the vehicle in the backward direction are switched by the dog clutch. Therefore, a shifting delay may occur during the so-called garage shifting. That is, the dog clutch is switched without applying the torque thereto. According to the teachings of Japanese Patent Laid-Open No. 2000-130548, therefore, a torque transmission to the dog clutch has to be cut off by disengaging the first clutch, and in this situation, the dog clutch is switched and then the first clutch is engaged. Therefore, it takes time to wait the torque applied to the dog clutch disappears, and the first clutch is engaged after conforming a fact that the dog clutch is switched. Thus, according to the teachings of Japanese Patent Laid-Open No. 2000-130548, the switching operation of the dog clutch and the engaging operation of the first clutch may be delayed inevitably to deteriorate shifting response during the garage shifting.
According to the teachings of Japanese Patent Laid-Open No. 2004-076876, the torque reversing mechanism establishes reverse stage when propelling the vehicle in the backward direction. The torque transmitted through the torque reversing mechanism is transmitted to the belt-driven continuously variable transmission, and the torque is further transmitted to the drive wheels. According to the teachings of Japanese Patent Laid-Open No. 2004-076876, therefore, the speed ratio under the reverse stage may be restricted to the ratio possible to be achieved by the continuously variable transmission.
The power transmission device taught by Japanese Patent Laid-Open No. 2005-308041 is adapted to reduce the torque applied to the belt-driven continuously variable transmission when propelling the vehicle in the forward direction. Therefore, the speed ratio of the gear train transmitting the torque when launching the vehicle is smaller than the maximum speed ratio that is possible to be achieved by the continuously variable transmission. For this reason, a total range of the speed ratio cannot be widened.